BASICS OF CROSSTALK, TRANSIENTS IN POWER SUPPLY LINES, OATS

1. EMI/EMC

2. Contents
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Crosstalk In Transmission Lines
Transients In Power Supply Lines
Surge voltages from lightning
Switching Transients
Electromagnetic Interference
Open Area Test Sites
Open Area Test Site Measurements

3. Crosstalk In Transmission Lines
• Crosstalk is the coupling of EM energy from one transmission
line to the another via:
- Mutual Inductance(magnetic field)
- Mutual Capacitance(electric field)

4. Mechanism of coupling
 The circuit element that represents this transfer of energy
are the following familiar equations
VLm
dI
Lm
dt
I Cm
dV
Cm
dt
 The mutual inductance will induce current on the victim line
opposite of the driving current (Lenz’s Law).
 The mutual capacitance will pass current through the mutual
capacitance that flows in both directions on the victim line.

5. Crosstalk In Transmission Lines

8. Summary of analysis:
 It is clear that superposition of inductive and
capacitive coupling is a function of
geometrical config., number of conductors
and terminal impedances.
 Thus the freq. range for which the analysis is
valid depends on no. of line conductors, crosssectional configuration of line and terminal
impedances Roi and RLi

9. Transients In Power Supply Lines

10. What are transient voltages?
•
"Transients or Transient Voltages“ is
generally called as "surges" or "spikes".
•
Transients are momentary changes in
voltage or current that occur over a short
period of time.
•
This interval is usually described as
approximately 1/16 (one sixteenth) of a
voltage cycle

11. • Voltage transients normally last only about 50
microseconds .
• Current transients last typically 20
microseconds according to the ANSI C62.411991 which is the standard for transients in
facilities operating under 600 Volts.
• Transients may be of either polarity and may
be of additive or subtractive energy to the
nominal waveform.

12. Characteristics of the Transient Voltage
Waveform
a) Oscillatory transient
b) Impulsive transient

13. Transients In Power Supply Lines
 Transient over-voltages (probably as a result of terrestrial
phenomena such as lightning.
 Radiation from strong radar/radio/communication transmissions
within the vicinity, which are picked up by the power
transmission lines.
 Sudden decrease or increase in the mains voltage (caused by
the switching of low impedance loads).
 Burst of high frequency noise (probably due to switching of
reactive loads).

14. Surge voltages from lightning

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

A nearby lightning strike to objects
Lightning ground-current flow
The rapid drop of voltage
A direct lightning strike to high-voltage
primary circuits.
 Lightning strikes the secondary circuits
directly.

15. Switching Transients
• Minor switching near the point of interest
• Periodic transients (voltage notching) that occur
each cycle during the commutation in electronic
power converters
• Multiple re-ignitions or re-strikes during a switch
operation
• Major power system switching disturbances
• Various system faults, such as short circuits and
arcing faults

16. Electromagnetic Interference

17. Electromagnetic Interference
• Undesired or unintentional coupling of
electromagnetic energy from one equipment
(emitter) to another equipment (receptor).

18. The so called source-path-receptor model suggests
that electromagnetic interference can be prevented in
one of three ways:
 Suppress emissions at the source.
 Interrupt or reduce the efficiency of the
path.
 Make the receptor immune to emissions.
coupling
One of the greatest difficulties in diagnosing interference is determining
exactly which coupling path interference is following to the receptor.
Coupling paths are typically classified as belonging to one of four general
classes.

19. 1. Conductive coupling
 Conductive coupling occurs when the path of
interference between the source and the
receptor is formed by a conducting body.
• By means of a power cord, interface
cables, antenna input terminals, ground
returns, or
• Unintentional external conductors such as
metallic cases or housings.

20. 2. Radiation coupling
 Radiation/electromagnetic coupling occurs when the
path of interference lies through free space, or some
other non-conductive medium.
 Radiation coupling usually occurs when distances
between the source and receptor are on the order of
several wavelengths.
 Because of this wide separation, the source is usually
not affected by the presence of the receptor.
 Radiation fields decay as 1/R for points far away from
the source.

21. 3) Induction coupling
 Inductive/magnetic coupling is associated with
near-fields, in a region where the magnetic
field is dominant.
 Occurs when distances between the source and
receptor are much less than a wavelength.
 Due to the relatively small separation, the
presence of the receptor can affect the behaviour
of the source, referred to as mutual coupling.

22. 4) Capacitive coupling
 Capacitive/electric coupling occurs when the
source and receptor are less than a
wavelength apart.
 Here the electric field is dominant.
 It occurs in the presence of high impedance to
ground and is more predominant at higher
frequencies.

23. Open Area Test Sites & Measurements

24. Open Area Test Sites & Measurements
Here we are going to study about
 Open Area Test Site
 Open Area Test Site Measurements
 Measurement of RE
 Measurement of RS
 Measurement precautions

25. OPEN -AREA TEST SITE
• The shape and size of the open-area test site will need
to be appropriate to avoid scattered signals.
• ANS recommend that Sc ≤ Sd -6dB -----(1)
where Sc & Sd are the scattered signal from obstructions
located at the boundary of open-area test site and the
direct signal between EUT and the Tx/Rx antenna.

26. OPEN AREA TEST SITE MEASUREMENTS
 The measurements of radiated emissions (RE) and radiated susceptibility
(RS) of an equipment constitute the two basic EMI and EMC
measurements.
 The purpose of radiation susceptibility testing is to determine the
degradation in equipment performance caused by externally coupled
electro magnetic energy.
 Open site measurement is most direct and universally accepted standard
approach for measuring radiated emissions from an equipment or the
radiation susceptibility of an equipment.

27. Measurement of RE

EUT is switched on

The receiver is scanned over the specific frequency range

It measures electromagnetic emissions from the EUT

It determine the compliance of these data with the stipulated specifications.
EUT
Power line
filter
Power
source
calibrated receiver/
field strength meter
Power
source

28. Measurement of RS
1) EUT is placed in an electromagnetic field created with the help of suitable radiating
antenna.
2) The intensity of the electromagnetic field is varied by varying the power delivered to
the antenna by the transmitter amplifier
3) Performance of EUT are then observed under different levels of electromagnetic
field intensity.
EUT
Power line
filter
Power
source
Transmitter
Power line
filter
Power
source

29. Test Antennas
• A convenient approach to illuminate an equipment under test with known
field strengths is to used exact half wave length a long dipoles at fixed
frequencies.
• This arrangement is superior when compared to connecting a test antenna
to a signal source using co-axial cable that might distort the field pattern.
Antenna Type
Frequency, MHz
Rod antenna
1 - 30
Loop antenna
1 – 30
Biconical antenna
30 – 220
Dipole antenna
30 - 1000
Log periodic antenna
200 -1000
Conical log spiral
200 – 10000
Wave guide horn
Above 1000

30. Examples of test antennas

31. Measurement Precautions
1) Electro magnetic environment
According to ANS, it is desirable that the conducted and radiated ambient radio noise
and signal levels measured at the test site with the EUT de-energized, be at least 6 dB
below the allowable limit of the applicable specification.
2) Electro magnetic scatterers
One method for avoiding interference from underground scatters is to use a metallic
ground plane to eliminate strong reflections from under ground sources such as
buried metallic objects.
3) Power and cable connections
The power leads used to energize the EUT, receiver and transmitter should also pass
through filters to eliminate the conducted interferences carried by power lines.